Return type inference broken for types with static members in ts 2.5; later fixed

[alexeagle]

Angular's dependency injection looks something like this:

interface Type<T> extends Function {
  new (...args: any[]): T;
}

function get<T>(token: Type<T>, notFoundValue?: T): T {
  return null as any as T;
}

class Token0 {
  a = 1;
}
class Token1 {
  static a = 1;
}

let t0 = get(Token0);  // t0 always inferred as Token0
let t1 = get(Token1);  // TS 2.5.3: t1 inferred as `{}` 
                       // TS 2.6.1 t1 inferred as `Token1`

As stated in those comments, starting in TS 2.5 we get the wrong type when a user asks the Injector for a Token with a static member. It's later fixed in TS 2.6.

Workaround is to give an explicit type argument to get, eg Angular users should repeat the type like Injector.get<Token1>(Token1)

There may be no action required here other than asking users to upgrade to TS 2.6 - but sadly Angular doesn't support it yet because we need to upgrade all the places we depend on first. Filing this issue mostly so we have a place to point to in the Angular changelog.

[aluanhaddad]

@alexeagle you could work around this temporarily by using the following signature for get.

declare function get<T>(token: new (...args: any[]) => T, notFoundValue?: T): T;

where in 2.5.3

// tslint:disable
interface Type<T> extends Function {
  new(...args: any[]): T;
}

declare function get<T>(token: new (...args: any[]) => T, notFoundValue?: T): T;

class Token0 {
  a = 1;
}
class Token1 {
  static a = 1;
}

let t0 = get(Token0); // Token0
let t1 = get(Token1); // Token1

[alexeagle]

Thanks for the suggestion @aluanhaddad !
It turns out it's not as simple as my repro: https://github.com/angular/angular/blob/69c53c3e03c7a8bb2260496c9aa32925cd2d59b4/packages/core/src/di/injector.ts#L60
changing the declaration of get causes some callsites to fall through to the second, any-typed overload. So it has some ripple effects in Angular's code. Maybe this could still be patched up to work.
/cc @IgorMinar

[RyanCavanaugh]

@DanielRosenwasser what does "In Discussion" mean here? Is there a concrete suggestion to be considered?

[DanielRosenwasser]

@RyanCavanaugh the suggestion is backporting whatever changes were applied to 2.5.x, which we don't typically do. The bug is already fixed, which is why I didn't mark it as such.

[aluanhaddad]

@alexeagle you should be able to add an overload that takes the existing types to preserve the current behavior. The new signature will need to be declared first since it is more specific.

abstract get<T>(token: new (...args: any[]) => T, notFoundValue?: T): T;
abstract get<T>(token: Type<T> | InjectionToken<T>, notFoundValue?: T): T;